The present invention refers to a photovoltaic panel, to the relative production process and to a plant for carrying out such a process.
Awareness of environmental problems connected with current main sources of energy (petrol, natural gas, coal, uranium) and the consciousness of their progressive depletion have pushed towards the research and exploitation of alternative sources over the last decade.
Amongst these, photovoltaic power generation seems to be the least intrusive for producing electromotive force and is applied already today by exploiting the properties of silicon.
The operation of a photovoltaic panel is based upon the homonymous effect which occurs when an electron in the valence band of a (generally semiconductive) material passes to the conduction band due to the absorption of a sufficiently energetic photon, incident on the material.
In particular, when a luminous flux on the other hand, hits a crystal lattice of a semiconductive material, a certain number of electrons are elevated to the conduction band corresponding to the same number of holes which pass to the valence band. In this way charge carriers are made available and can be used to generate current.
The photovoltaic panel is therefore made by placing a material which by atomic structure attracts electrons (and takes on negative charge) in contact with a material which tends to repel them (and takes on positive charge) thus obtaining an n type structure (with an excess of electrons) and a p type structure (with an excess of holes), respectively.
In both of the aforementioned materials, the electrical conduction must normally be stopped, but it must be easily activated by feeding energy to the valence electrons (characteristic of semiconductors) through photon radiation.
When an electron receives a sufficient amount of energy, it begins to move in the semiconductive material leaving the place it occupied empty (electron hole) that will be filled by adjacent electrons, generating a behavior similar to that of a positively charged moving particle.
Due to the contact between the two materials having different electrical affinity, the electron will tend to shift in a preferential direction generating an electric current (the corresponding hole shifts in the opposite direction) which can be collected by conductors and advantageously used.
Only if the electron-hole pair reaches the junction between the two semiconductors is it definitively disassociated and it generates electrical current. In general, a photovoltaic generator structure according to the prior art comprises, in the order illustrated in FIG. 1:                a first protective layer 1;        a conductive layer for collecting electrons (negative pole) 2;        a negative semiconductive layer 3 (n);        a positive semiconductive layer 4 (p);        a conductive layer 5 for collecting electron holes (positive pole); and        a second protective layer 6.        
The photovoltaic panel illustrated in FIG. 1 has therefore the first protective layer 1 in glass, the conductive layer 2 in turn made through a metallic mesh 2′ embedded in a transparent conductive layer 2″. The p and n semiconductive layers 3 and 4 respectively are conventionally made through “doped” silicon respectively with elements of group III and V of the periodic system of elements.
Since the photons must reach the semiconductor, at least one of the protective layers and the adjacent conductive layer must be transparent to light.
The conventional photovoltaic cells use silicon doped, for example, with boron and phosphorus to make the two active intermediate layers (semiconductors) and conventional metal conductors for the most outer layers. One of the two conductive layers is formed by a metallic wire net to allow silicon to be illuminated.
In the conventional production process, the silicon foil (wafer) is obtained by slicing a suitably pure ingot, later doped by diffusion in high temperature ovens and assembled with the different layers that form the cell.
The process is costly and laborious and justified only in view of environmental and strategic reasons, obtaining institutional incentive for this reason or due to the lack of other energy sources.
In order to optimize the use of silicon and make the production process more cost effective, technologies have been developed in which the various components are obtained by deposition of thin films (of the order of nanometers nm, or of micrometers μm).
The silicon thus obtained is amorphous; it has a shorter life and a lower yield than the crystalline silicon of the wafers, but the reduction in cost makes up for the drawbacks.
Solutions using “conjugated structure” polymers, that is polymers in which the carbon atom chain C, contains at least partially regular configurations of double covalent bonds (σ, π), have been proposed and in some cases made.
In these polymers, the electron forming the bond π is, according to the most accredited model, delocalized in an electron cloud surrounding the molecule and can easily be excited to the conduction band generating a pair (electron-hole).
The particular chemical composition and/or the presence of suitable doping elements, gives the polymer the property of being an electron-donor (like in the case of poly (2-methoxy, 5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV)) or an electrons acceptor (like in the case of [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM)) allowing photovoltaic semiconductive pairs to be created.
Examples of photovoltaic cells made with conductive polymers are described in technical/literature and/or form patent prior art.
For example, cells are known comprising the following layers:
ITO/PBI/P3HT/Au, (ITO: Indium Tin Oxide; the rear electrode is a thin gold film);
PEDOT: PSS/MEH-PPV/PCBM/Al (PEDOT: PSS conductive polymer; and
Polyethylenedioxythiophene/polystyrenesulfonate, the rear electrode is made of aluminum).
One of the conductors must be transparent in order to allow the semiconductive films to be irradiated.
Currently, ITO (ITO: Indium Tin Oxide) is used.
A suitable electron collecting system must then be associated with this, normally a small-meshed wire net of copper with thin wires.
All of this is closed between protective films to avoid the components from oxidizing. Organic or metal materials are normally used.
These polymers however, cannot withstand high temperature transformation processes, customary for common plastic materials, they have a relatively low energy yield and a relatively short life (always compared to silicon).
The possibility of making solar cells with cheaper procedures and even with continuous processes is a constant goal of the scientific community and industry.
According to the prior art different proposals can be found, for example:
U.S. Pat. No. 4,260,429, concerns a transparent electrode formed by embedding a metallic wire net in a charged polymer, typically a fluoropolymer charged with elements according to the semiconductive type it is to be coupled with:                for Silicon (Si), the conductive particles are Au/Si, Au/Ge, Ge, Al/Si, Al;        for Gallium Arsenide (GaAs), the conductive particles are Au/Ge, Au/Sn, Sn;        for Cadmium Sulphide (CdS) the conductive particles are In, Nb, Ni, Ti/Al, Hg; and        for Copper Sulphate Cu2S the conductive particles are Au, graphite, Cu, conductive Carbon Black, Pb/Sn.        
According to U.S. Pat. No. 4,479,027 a photovoltaic film is obtained by laminating and coupling metallic and ceramic Silicon-based strips.
U.S. Pat. No. 6,184,057 B1 teaches a production process of a photosensitive film obtained through successive deposition of layers.
In a publication by Zhang et al. of 2002, a film is proposed made through successive depositions PEDOT:PSS/MEH-PPV/PCBM/Al (aluminum cathode deposited through vaporization) having high flexibility.
There are also partial solutions to make or optimize parts of the cell.
Glass is the conventional solution for protecting photovoltaic cells on the side exposed to solar radiation. However it does have limited mechanical characteristics.
More resistant alternatives are:                Polycarbonate (PC) (it has low weatherability and is oxygen permeable)        Polyvinyl fluoride (PVF) (DuPont); and        Ethylene/tetrafluoroethylene copolymer (ETFE) (DuPont).        
The rear protection can be made from a metal support but solutions involving single or multilayered polymers or polymeric blends have also been proposed:                multilayer PVF/ETFE/PVF;        single layer or multilayer PA (polyamide), PET (polyethylene terephthalate), PTFE (polytetrafluoroethylene);        multi-layer PC (polycarbonate) and fluorinated polymers, possibly with addition of an EVA (Ethylene-vinyl acetate) layer (BASF); and        PA (polyamide)/ionomers mixture (DuPont).        
The processes described in literature for making the active part of a photovoltaic cell, that is the layers in which light radiation is converted into electrical potential difference, are carried out:                through deposition following vapour condensation by direct synthesis on the support (chemical reaction);        through condensation of a solution through removal of the solvent and depositing of the solute;        through deposition following printing (ink jet, lithography or others); and        through electrodeposition on a conductive substrate.        
The procedures currently known, relative to the layers that form the entire cell (electrodes, semiconductors, protection, and similar) alternatively or in combination foresee:                the subsequently deposition of the layers through total or partial coating of the layers previously created; and        the preparation of semi-finished products subsequently coupled by lamination.        